1. Field of the Invention:
The present invention relates to an optical amplifier, and more particularly to an optical amplifier which is capable of emitting a beam of light having high energy and is suitably used as a light source for an optical recording apparatus for recording information such as a character on a recording medium.
2. Description of the Related Art:
As an apparatus for recording information such as a character on a recording material by means of a beam of light, a laser computer output microfilmer (lasercom) which directly records information such as a character on a recording material including a microfilm by effecting scanning with a laser beam on the basis of information outputted by a computer is known (Japanese Patent Laid-Open No. 67722/1975). This lasercom has an argon laser for oscillating a laser beam, an optical modulator for optically modulating the laser beam in response to the character information, a rotating polygon mirror for polarizing in the horizontal scanning direction the laser beam modulated by the optical modulator, and a galvanometer provided with a polarizing mirror for polarizing in the vertical scanning direction the light reflected from the rotating polygon mirror. The arrangement is such that the laser beam outputted from the optical modulator is used to effect two dimensional scanning on a recording material via a scanning lens by means of the rotating polygon mirror and the galvanometer, thereby recording information such as a character on the recording material.
With the above-described lasercom, however, since an argon laser which cannot be on-off controlled, an optical modulator and the like are necessary, a proposal has recently been made to use a semiconductor laser instead of the argon laser. As such semiconductor lasers, SDL-2410, SDL-2420 (brandnames of Spectra Diode Labs Inc.) Series and the like are available. With respect to laser beams emitted from laser oscillating regions of such a semiconductor laser, it is known that a phase difference exists, and that, when the phase difference between laser beams emitted from the laser beam oscillating regions is 180.degree., two lobes are formed in the direction along a surface of a p-n junction in a far field pattern. Accordingly, even if a semiconductor laser which forms these two lobes is used as a light source for recording on a recording material, the laser beam fails to focus into one spot, so that it is impossible to realize an optical system having a high degree of resolution. In particular, in cases where character information or the like is recorded with dots on a microfilm by using a laser beam, resolving power on the order of 3,360 dots/7.2 mm is required, so that it is necessary to effect recording with dots with a very high degree of accuracy, with the result that the aforementioned two lobes present a problem.
For this reason, a proposal has hitherto been made to use one lobe by cutting the other (Appl. Phys. Lett. 41 (12), Dec. 15, 1982). In addition, Japanese Patent Laid. Open No. 98320/1987 discloses an arrangement in which the two lobes are separated from each other after a laser beam is converted into a parallel bundle of rays, and the two lobes are then combined into one by using a reflecting mirror, a half-wave plate, and a polarized beam splitter.
However, with the aforementioned conventional optical system in which one lobe is cut, there is a problem in that since one half of the beam for forming a lobe is cut, the light intensity of the laser beam emitted in the direction along the surface of the p-n junction is reduced to one half or thereabouts, so that the efficiency is poor. Consequently, the application of this optical system is difficult with respect to a recording material which requires high energy in recording as in the case of a heat. mode recording material such as a laser direct recording film (LDF).
If the above-disclosed optical system in which the lobes are combined is used, it is difficult in practice to obtain a completely parallel bundle of rays, and if the optical path lengths of the individual separated lobes are not equal, the position of a beam waist at the time when the beam is focused by a final lens becomes offset from an optical axis, thereby making it difficult to effect focusing at a high energy density.